what are the main structures that make up microbial cells and where are they found
- membrane enclosed organelles are uncommon
- ribosomes are located in the cytoplasm
- inclusions may also be found in the cytoplasm
(storage ex. polyphosphate, sulfur)
(metabolism ex. carboxysomes, anammoxosomes)
(motility ex. magnetosomes, gas vacuoles) - genome is located in nucleoid, no nucleus (transcription and translation at the same time), takes up larger fraction of cytoplasm
describe the types of microbial genomes
- genome includes chromosomal and extra chromosomal DNA
- single circular chromosome most common (monoploid)
-> 130kbp -14Mbp, around 3-5 Mbp is typical for free living cells
-plasmids (typically circular) encode non-essential functions
-> shorter than chromosomal DNA (30 genes on average)
-> an organism can have several different plasmids
describe the different types of filaments that protrude from the microbial cell
- fimbriae (help attarch to surface) and pili (help transport genetic maserial) (thin appendages, 3-10nm thick)
- flagella ( 20nm thick, up to 20um long),
-> composed of protein
-> used for swimming and swarming motility through fluid
describe the different types of flagellar arrangement
monotrichous - 1 flagella, if it is on the end = polar
amphitrichous - 1 flagella on both sides
lophotrichous - cluster of flagella at one or both ends, polar
peritrichous - many flagella all over the surface of the cell, spread evenly
list the components of the flagellum and the steps in which they are assembled
filament - flagellin protein and cap
hook - wider than filament
basal body - contains the motor
-> gram neg = 4 rings (L, P, MS, C)
-> gram pos = 2 rings (inner and outer)
- rings anchor into cell wall
- complex process involving 20-30 genes
- filament grows from the tip with flagellin subunits self-assembly
-Flagellin subunits travel through the hollow flagellum and attach to the growing tip. Their attachment is directed by the filament cap protein.
explain how flagella rotate
- flagellum operates like a motor
-> MS ring and C ring are the rotor - > MotA and MotB are the stator (anchored and dont move)
-> torque is transferred to the filament driving rotation - PMF (proton motor force) is used to fuel the motor
-> MotA and MotB create a channel through which protons can flow = causes the flagellum to rotate
-> rotation speed is controlled by the magnitude of the PMF
Explain which direction flagella travel based on the rotation
- run = smooth swimming movement CCW
- tumble (CW rotation), reorients the cell
- run stop
- run reverse flick
- speed is 10-100 um/s
- swarming = cells move in unison across a moist surface
describe the cytoskeleton on microbial cell structure
- homologs or analogs to eukaryotic cytoskeleton proteins exist in microbes
- determine cell shape
- cell division
- intracellular organization
describe the proteins found in cytoskeleton and the eukaryotic homolog
- caulobacter spp. (have been used as a model organism to study bacterial cytoskeleton)
- crescentin -> induces curvature in vibrios (found on inside of the curve) -> eukaryotic = intermediate filaments
- MreB -> determines cell shape in rods (found in bends along the length -> eukaryotic = actin
- FtsZ -> involved in cell division (forms during cell division) -> eukaryotic = tubulin
what are the outer microbial cell structures
- cell envelope -> encloses the cell, more complex than many eukaryotic cells
- capsule -> polysaccharides and or protein layer (to protect microbes and help them succeed in their environment, helps avoid immune cells)
- S layer -> slimy glycoprotein or protein layer (more organized, and more proteins, thin, protect from pH changes, pathogens attaching to cell surface)
- cell wall - found in all
- plasma membrane - found in all
describe the role of plasma membrane
- separates the cell from the outside world
- selectively permeable barrier
- site of many metabolic reactions and energy transduction
- site of lipid and cell wall synthesis
what is the structure of the plasma membrane
- lipid bilayer
- phospholipids -> hydrophobic lipid tails inside, hydrophilic head groups outside
- hopanoids -> control cellular domains, hydrophobic and rigid
- proteins -> integral membrane proteins
-> peripheral membrane proteins
-> organized into functional microdomains
lipid + protein = half of membrane
explain the lipid bilayer in more detail
- phospholipids
- amphipathic
- membrane fluidity is determined by lipid composition and length
- ester linkage between fatty acids and glycerol in bacteria
- the membrane must always be fluid
- hopanoids
-> similar structure and function to cholesterols - > organize membrane into microdomains
-> spatial organization of functions
explain bacterial vs archaeal lipids
archaeal
- lipid bilayer or monolayer
- ether linkages rather than ester linkages
- isoprene units (more variability help in temp)
- pentacyclic rings increase rigidity in very high temps
explain selective permeability
permitted = small uncharged, polar molecules and gasses and some larger non-polar/hydrophobic molecules
-> urea, ethanol, H2O, O2, CO2, H2, benzene
excluded = ions, charged, larger polar molecules and large molecules
-> glucose, H+, Na+, peptides
- membrane proteins can be used to transport selected molecules across the membrane
- allows cells to control what goes in and out and create concentration gradients
what is diffusion
- net movement of molecules from a region of high concentration to low concentration
- passive process, no energy required
applies to -> molecules that can pass through the lipid bilayer
-> molecules which can pass through transport proteins
what is passive transport
- facilitated diffusion
- the movement of particles across the plasma membrane down their concentration gradient, facilitated by transport proteins
- channel or carrier mediated
what is active transport
- the movement of particles across the plasma membrane against their concentration gradient, facilitated by transport proteins
- requires an input of energy
-> primary active transport
-> secondary active transport
-> group translocation
what is primary active transport
- energy source = ATP
- mediated by uniporters (ABC transporters, ATP binding cassette)
- ATP binding domain on cytoplasmic side hydrolyzes ATP when solute enters cell
what is secondary active transport
- energy source = ion gradients (Na+, H+)
- mediated by cotransporters
- symporter = transferred in same direction
- antiporter = transferred in opposite directions
lactose permease (LacY) = transport of lactose and H+ across the membrane
what is group translocation
energy = high energy molecule like PEP
- phosphoryl system transfers Pi from PEP to incoming sugar
- enzyme 1 accept Pi from PEP and transfers it to HPr
- HPr transfers Pi to enzyme 2, which transfers it to a specific sugar at the transporter
- enzyme 2 is specific for a given sugar
intracellular membrane systems
- certain microbes have extensive internal membrane systems to increase surface area of plasma membrane
- usually associated with metabolic processes occurring at the membrane (phototrophs, nitrifers)
- bacteria and archaea do not have extensive membrane bound organelles like eukaryotes
what is the function of the cell wall
- almost all microbes have cell walls
- maintains cell shape
- protects from osmotic lysis
- protects from toxic substances
- contributes to pathogenicity
what are the common components of bacterial and archaeal cell envelopes
- one or more membrane
- Peptidoglycan
- S layer
- Capsule
- protein layers
(1) The most common envelope
is a simple S-layer.
(2) Some S-layers are covered with an additional protein sheath or a carbohydrate layer.
(3)Other archaea have a carbohydrate layer
beneath the S-layer or in place of an S-layer (4). (5) A few have a double membrane.
